The present invention relates to digitally watermarking media, and in particular, to digitally watermark a compressed video stream such as a MPEG stream.
When delivering digital media, it is highly desirable to the content owner to be able to digitally mark (“watermark”) each copy of the digital media to identify the origin of the content, e.g., in order to track down and discourage copiers.
Many standards are known for coding multimedia streams such as video streams. Common transform coding methods, such as Motion JPEG, MPEG1, MPEG2, ITU-T-261, etc., encode as follows. For motion video, an image is divided into blocks, e.g., 8 by 8 or 16 by 16 groups of pixels. Each image is classified as interframe or intraframe. Motion estimation determines a set of frame-to-frame motion vectors, and for interframe coding, images are typically motion compensated according to the determined motion vectors. Interframe images are formed post-motion compensation. The blocks of the image are transformed, e.g., by a discrete cosine transform (DCT), and the transform coefficients of the block are quantized, e.g., such that some higher frequency components are quantized to zero. The quantized transform coefficients of each block are then coded along a specified path according to a variable length coding. The DC component is typically separately encoded. Newer transform coding techniques include the H.264/MEG-4 AVC video coding standard/draft standard that defines 4×4 blocks and a DCT-like 4×4 integer transform.
It is known to use MPEG encoding techniques to encode a watermark or other identifying information in a video stream. Such known attempts fall into three basic categories:                (1) Modification of existing images to encode watermark data, e.g., into the high frequency regions of motion vectors such that there is no significant perceptible visual degradation of the image.        (2) Modification of existing motion vectors to degrade the image in such a method that the watermark is barely perceptible, but cannot be viewed without a secondary program to undo the watermarking        (3) Modification of a random-like, but known portion of the media stream, producing degradation of portions of the images.        
Such prior art methods are discussed in various references. One good source is the Watermarking FAQ, available on the Web at http://www.watermarkingworld.org/faq.html.
For example, Bodo Yann, Nathalie Laurent, Jean-Luc Dugelay: A scrambling method based on disturbance of motion vector, Proceedings, ACM Multimedia 2002, pp. 89-90, Juan les Pins, France describes a method in which video data are scrambled efficiently by disturbing a subset of motion vectors. The interest of this approach is its ability to scramble a video while maintaining a certain visibility. Effectively it permits the use of considerable levels of security in order to choose the level of perceptibility of the video.
Several prior art methods also are mentioned in “Research on digital watermarking at Aristotle University of Thessaloniki, November 1997” available from Aristotle University of Thessaloniki, Thessaloniki, Greece, Also at http://poseidon.csd.auth.gr/signatures/report.html
See also Oscar Divorra Escoda, Rosa Maria Figueras i Ventura, Eric Debes, Touradj Ebrahimi, “Influence of a Large Image Watermarking Scheme Parallelization on Possible Attacks” in proceedings of the SPIE's Annual Meeting on Optical Science and Technology, San Diego, Calif., July-August 2001. Available at http://www.eric.debes.net/SPIEWMAttacks.pdf
U.S. Pat. No. 5,960,081 to Vynne, et al., titled EMBEDDING A DIGITAL SIGNATURE IN A VIDEO SEQUENCE describes embedding a digital signature into the x- and y-coordinates of motion vectors.
See also U.S. Pat. No. 6,381,341 to Rhoads titled WATERMARK ENCODING METHOD EXPLOITING BIASES INHERENT IN ORIGINAL SIGNAL.
A good survey article is F. Hartung, M. Kutter: “Multimedia Watermarking Techniques”, Proc. of IEEE, pp. 1079-1107, July, 1999.
A tutorial introduction to the overall field of watermarking is S. Mohanty, Digital Watermarking: A Tutorial Review, Department of Computer Science, University of South Florida, Tampa, Fla. 33620. Prepared in 1999 at the Indian Institute of Technology, Bangalore, India. Available on the Web at http://www.csee.usf.edu/˜smohanty/research/Reports/WMSurvey1999Mohanty.pdf, and also at http://citeseer.ist.psu.edu/mohanty99digital.html.
See also M. Wu and B. Liu, “Date Hiding in Image and Video, Part I Fundamental Issues and Solutions”, IEEE Trans Image Processing, June 2003, pp 685-695.
Even though there is much prior art in the field, there still is a need in the art for a method of watermarking a media stream that does not require wholesale re-encoding of the content. There further is still a need in the art for a method that does not (substantially) affect the size of the content. There still is a need in the art for a method that is difficult to detect by a consumer. There also is still a need in the art for a media digital watermarking method that does not affect the content presentation at all. For example, there is a need in the art for a method applicable to MPEG streams that results in a watermarked stream that is a valid MPEG stream that looks (and sounds) identical to the original stream being watermarked. There further is still a need in the art for a method that can be implemented with relatively low overhead, e.g., by a content distributor, so that such a content provider can add a distinguishing digital “watermark” without undue processing time.